tree.ts

type Tree<T> = {left: Tree<T>, right: Tree<T>, value: T}

function reduceTree<T,A>(tree: Tree<T>, initialAcc: A, reducer: (A,T) => A): A{
	if(tree == null){
		return initialAcc;
	}
	var withNodeValue = reducer(initialAcc, tree.value);
	var withLeftBranch = reduceTree(tree.left, withNodeValue, reducer);
	var withRightBranch = reduceTree(tree.right, withLeftBranch, reducer);
	return withRightBranch;
}

type AverageAcc = {sum: number, count: number}

function average(tree: Tree<number>): number{
	var acc: AverageAcc = {sum: 0, count: 0};
	function reduce(acc: AverageAcc, val: number){
		return {sum: acc.sum + val, count: acc.count + 1};
	}
	var result = reduceTree(tree, acc, reduce);
	return result.sum / result.count;
}

var testTree: Tree<number> = {
	value: 1, 
	left: {
		value: 2,
		left: null,
		right: null
	}, 
	right: {
		value: 4,
		left: null,
		right: {
			value: 5,
			left: null,
			right: null
		}
	}, 
};

console.log(average(testTree));